![]() For example, "a 25-foot cliff was thrusted into the air from west Seattle out to Puget Sound. "These are four shallow faults that had shown evidence of having ruptured roughly 1,000 years ago in a cluster of earthquakes called the millennial cluster," Black said. ![]() Scientists have been uncovering shallow faults in the region since the 1960s, when the Seattle Fault was first discovered, followed by the Saddle Mountain Fault, which runs along the eastern foothills of the Olympic Mountains, and the Tacoma and Olympia faults. Regional hazard models, used to develop engineering design and policies, don't currently reflect this possibility - but should, said paper first author Bryan Black, an associate professor of dendrochronology in the UArizona Laboratory for Tree-Ring Research. While earthquakes are not new to the Pacific Northwest, the study identified that events on these shallow faults are linked to each other in some way, either by connections underground or by one fault transferring stress to the other. Shallow faults typically result in more violent and focused shaking than earthquakes generated from other geological configurations. ![]() The ancient quake was either the result of all the shallow faults in the region rupturing together to produce an estimated 7.8-magnitude earthquake or - like in Turkey and Syria - twin quakes that occurred back-to-back with estimated magnitudes of 7.5 and 7.3, researchers found. The results were published in the journal Science Advances. Their findings mean that a repeat event has the potential to again shake the region that is now home to over 4 million people, including Seattle, Tacoma and Olympia. Tree rings helped pinpoint that the seismic event occurred in late A.D. Similar shallow faults ruptured about 1,000 years ago in the Puget Lowlands in western Washington, according to new University of Arizona-led research.
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